Ultrasonic welding
Ultrasonic welding is a solid-state joining process that employs high-frequency ultrasonic vibrations, typically in the range of 20–40 kHz, to generate frictional heat at the interface of two materials, enabling their fusion or diffusion bonding under applied pressure without melting the bulk material.[1] This technique is widely used for thermoplastics, where vibrations cause localized melting at the joint, and for thin metals, where it promotes atomic diffusion for a metallurgical bond.[2] Developed initially in the 1940s and patented for thermoplastics in 1965 by Robert Soloff and Seymour Linsley at Branson Instruments, ultrasonic welding has evolved into a rapid, energy-efficient method suitable for automated production.[3] The process begins with a transducer converting electrical energy into mechanical vibrations, which are amplified and directed to the workpiece via a horn or sonotrode, while an anvil holds the parts in place under controlled force.[4] For thermoplastics, compatible materials with similar melt temperatures (within approximately 6°C) are essential, often featuring joint designs like energy directors—a triangular protrusion that concentrates energy for efficient melting—or shear joints for stronger bonds.[5] In metal welding, the vibrations scrub the surfaces to remove oxides, facilitating direct metal-to-metal contact, with typical amplitudes of 1–25 µm and weld times under 1 second.[1] Process parameters such as frequency, amplitude, pressure, and time are optimized to achieve consistent joint strength, often using methods like response surface methodology.[2] Ultrasonic welding offers distinct advantages, including its speed (cycle times of 0.1–1 second), cleanliness (no fluxes, fillers, or solvents required), and versatility across materials like aluminum, copper, and dissimilar combinations such as metal-to-plastic.[1] It is particularly effective for heat-sensitive components, minimizing distortion or recrystallization in metals like molybdenum and tungsten.[4] However, limitations include challenges with thicker materials (>3 mm for metals) or high-strength alloys, potential for nonuniform coupling, and the need for precise surface preparation.[4] Applications span multiple industries, including automotive (e.g., battery tab welding and plastic assemblies), electronics (circuit board connections), medical devices (hermetic seals for implants), packaging (foil and film sealing), and textiles (nonwoven fabric bonding).[2] In recent advancements, it has been adapted for thermoplastic composites and battery manufacturing, including continuous welding for aircraft fuselage components in 2024, supporting lightweight structures and electric vehicle production.[6] Overall, its adoption continues to grow due to demands for sustainable, high-precision manufacturing processes.[2]Background
History
Ultrasonic welding was first developed in the late 1940s, with the first U.S. patent for ultrasonic metal welding granted in 1960 to Aeroprojects Inc., initially applied to splicing thin metal foils such as aluminum, half the thickness of a human hair, for applications in aerospace and electronics.[7] This innovation addressed limitations in traditional welding methods for delicate, non-ferrous materials, marking the technology's debut in industrial contexts.[8] The extension to plastics followed closely, revolutionizing thermoplastic bonding. In 1963, Robert Soloff discovered the ultrasonic method for welding rigid thermoplastic parts while experimenting with an ultrasound probe at Branson Instruments, later founding Sonics & Materials Inc. in 1969.[3] This led to U.S. Patent No. 3,224,916, awarded to Soloff and Seymour Linsley in 1965, for the ultrasonic welding of thermoplastics.[9] Commercialization accelerated in the late 1960s; Sonics introduced the first commercial ultrasonic plastic welder in 1969, coinciding with the assembly of the first all-plastic car using the process.[9] Additional patents, such as those filed between 1965 and 1967, refined the technique for broader plastic applications.[10] By the 1970s, ultrasonic welding evolved from low-power laboratory setups to robust industrial machines, fueled by the electronics industry's demand for precise, solvent-free joining of small components.[11] Standard 20 kHz systems emerged during this decade, enabling higher energy delivery for consistent welds in production environments.[12] The 1980s saw widespread adoption in the automotive sector, where the technology replaced mechanical fasteners for plastic assemblies, enhancing efficiency and strength.[7] Integration with robotic systems around 1985 further automated processes, improving precision in high-volume manufacturing.[13] In the 1990s, advancements focused on high-power ultrasonic welding for metals, broadening applications to thicker foils and dissimilar materials in battery and aerospace sectors.[14] This period solidified the technology's role in demanding industries, with innovations in power delivery enabling reliable joints without heat-affected zones.[15]Fundamentals
Ultrasonic welding is a solid-state welding process that employs high-frequency ultrasonic vibrations, typically ranging from 15 to 70 kHz, to produce frictional heat at the interface between two workpieces, facilitating a strong bond without melting the bulk material or requiring filler metals.[16] This technique relies on the application of mechanical oscillations perpendicular to the joint line, combined with a static clamping force, to concentrate energy at the faying surfaces.[17] The process is distinct from fusion welding methods, as it avoids widespread thermal distortion and preserves material properties through localized heating.[18] Key mechanisms differ by material type. In thermoplastics, viscoelastic heating predominates, where the cyclic deformation of polymer chains under vibration induces hysteresis losses, generating heat that softens the interface for interdiffusion and fusion upon cooling.[17] For metals, particularly thin sheets or foils, the process involves interfacial friction between asperities, which breaks oxide layers, coupled with localized plastic deformation that dissipates energy as heat and promotes metallurgical bonding; cavitation effects may also occur at microscopic scales, aiding in surface cleaning and deformation.[19] In both cases, no bulk melting occurs, but the interface experiences sufficient softening or flow to achieve coalescence under pressure.[16] Energy transfer begins with electrical input converted to longitudinal acoustic waves via a piezoelectric transducer, propagating through the welding tool to the interface.[18] These vibrations, with amplitudes typically 10-50 μm, are transformed into thermal energy through material-specific dissipation: hysteresis in viscoelastic polymers and frictional/plastic work in metals.[17] The efficiency of this conversion depends on resonance matching between the system components and the workpiece, ensuring maximal energy delivery to the joint.[20] Material suitability is governed by the ability to accommodate localized deformation without fracture. Thermoplastics like ABS and PVC are ideal due to their viscoelastic nature and the prerequisite of compatible melting points, ideally within about 10°C, for uniform heating and bonding.[17][5] Thin non-ferrous metals, such as aluminum sheets or foils typically up to 3 mm thick, weld effectively owing to their ductility, enabling plastic flow under vibration; ferrous metals or thicker sections are less amenable due to higher hardness and limited deformation.[16][1] The power input driving this process can be modeled for the vibrating horn asP = \frac{1}{2} m A^{2} (2 \pi f)^{2},
where P is the average power (in watts), m is the effective mass of the horn (in kg), A is the vibration amplitude (in meters), and f is the frequency (in Hz). This formulation arises from the harmonic motion dynamics of the ultrasonic stack, where the average power equals the product of the damping losses and the squared maximum velocity \omega A (with \omega = 2\pi f), assuming the effective damping equates to the inertial response of the horn mass at resonance. To derive it, consider the kinetic energy of the horn at maximum displacement: KE_{\max} = \frac{1}{2} m (\omega A)^{2}. Over one cycle, the energy dissipated to the interface equals this maximum, and multiplying by frequency f yields the average power, simplifying under resonant conditions where potential and kinetic energies balance without phase lag.[20]